Conventionally a method in which a high voltage (DC 6 to 8 KV) is loaded to a metal wire and electrification is executed by making use of generated corona has widely been used as an electrifying process in an electrophotographic process. In the corona discharge system as described above, however, corona products such as ozone or nitrogen oxide generated when corona is generated denature a surface of a light-sensing body, and promote shading or degradation of an image, and contamination of wire gives bad effects to a quality of an image.
On the other hand, there has been developed and put into practical use a contact electrifying system in which a voltage is loaded to an electrifying member in contact with a light-sensing drum for electrifying a surface of the light-sensing body. This electrifying system has the advantage that a voltage drop in the power supply unit and a quantity of generated ozone are relatively small, but the uniformity in electrification is substantially inferior to that in the corona discharge system.
To improve the uniformity in electrification, for instance, in Japanese Patent Laid-Open Publication No. 149668/1988 disclosing the "contact electrifying method", it is disclosed that the uniformity in electrification can substantially be improved by superimposing an AC voltage having an inter-peak voltage which is 2 times or more larger than that of an electrification start voltage (V th) when a DC voltage is loaded.
Also for improving the uniformity in electrification, for instance in Japanese Patent Publication No. 16033/1993 disclosing an "electrifying apparatus", it is disclosed that uniformity in electrification can substantially be improved with the electrifying apparatus comprising a conductive base to which a voltage is loaded, a first resistive layer provided at a position closer to an electrified body than this conductive base, and a second resistive layer provided at a position closer to said electrified body than this first resistive layer and having a larger volume resistivity than that of the first resistive layer, and having an area where a distance from said electrified body becomes larger and by loading a voltage having an inter-peak voltage which is 2 times or more larger than an electrification start voltage to the electrified body.
However, in the contact electrification method disclosed in Japanese Patent Laid-Open Publication No. 149668/1991, to superimpose a high AC voltage having an inter-peak voltage which is 2 times or more larger than an electrification start voltage (V th) when a DC voltage is loaded, an AC power supply source different from that for a DC power is required, which causes increase of the apparatus cost. Furthermore by consuming a quantity of AC power, there occur various problems such as generation of a large quantity of ozone or drop in durability of the electrifying roller or the light-sensing body.
Although these problems can be solved by loading only a DC voltage to the electrifying roller, but when only a DC voltage is loaded to an electrifying roller, problems as described below occur.
(1) At first, an electrifying roller was prepared, according to the method disclosed in Japanese Patent Publication No. 16033/1993, by covering a metal core rod having a diameter of 8 mm with urethane rubber as an elastic layer containing carbon dispersed therein and having an electric resistance of 10.sup.4 .OMEGA.cm and thickness of 4 mm, and then providing a surface layer having a thickness of 25 mm made from a cellophane sheet with a volumic resistivity of 10.sup.9 .OMEGA.cm, and only a DC voltage was loaded, when nonuniformity in electrification was generated. Nonuniformity was generated when carbon black was dispersed in synthetic rubber as a material for an elastic layer of the electrifying roller. Namely nonuniformity in the electrifying roller in this case is generated because of electric nonuniformity in the elastic layer due to carbon black dispersement fault in synthetic rubber.
(2) A DC voltage of 1.5 KV with negative polarity was loaded to this electrifying roller to electrify a light-sensing drum such as an OPC to (-) 800 V, and the electrified light-sensing drum was used. In this case an excessive current flew through pin holes in the light-sensing layer, and a voltage loaded to the electrifying roller dropped, which made it impossible to electrify the light-sensing drum.
(3) An electrifying roller comprising EPRM with carbon dispersed therein as an elastic layer having an electric resistance in a range from 10.sup.4 to 10.sup.5 .OMEGA.cm and a thickness of 3 mm, hydrin rubber as a resistive layer having an electric resistance in a range from 10.sup.7 to 10.sup.9 .OMEGA.cm and a thickness of 100 .mu.m, and carbon-dispersed nylon resin having an electric resistance in a range from 10.sup.7 to 10.sup.9 .OMEGA.cm and a thickness of 100 .mu.m was used, and only a DC voltage was loaded, and in this case, because dispersibility of carbon and EPDM was good, electric uniformity of the elastic layer is improved, but withstand voltage capability of the roller layer is rather poor, so that leak of electric charge through pin holes in the light-sensing layer often occurred.
(4) In the conventional type of elastic layer made of carbon and synthetic rubber, it is possible to adjust an apparent electric resistance by changing a quantity of added carbon, but it is difficult to simultaneously realize appropriate conductivity.
(5) Furthermore, electric uniformity of an elastic layer can be improved by increasing a quantity of carbon dispersed therein to lower an electric resistance of the entire layer (to 10.sup.4 .OMEGA.cm), but when used only with a DC voltage, the withstand voltage capability becomes lower.